This randomized phase II-R/III trial studies gemcitabine hydrochloride with or without erlotinib hydrochloride followed by the same chemotherapy regimen with or without radiation therapy and capecitabine or fluorouracil in treating patients with pancreatic cancer that was removed by surgery. Drugs used in chemotherapy, such as gemcitabine hydrochloride, capecitabine, and fluorouracil, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Erlotinib hydrochloride may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Radiation therapy uses high energy x-rays to kill tumor cells. Giving chemotherapy together with or without erlotinib hydrochloride and/or radiation therapy after surgery may kill any tumor cells that remain after surgery. It is not yet known whether chemotherapy is more effective when given with or without erlotinib hydrochloride and/or radiation therapy in treating pancreatic cancer.